Chris Stephen: Nato-aided successes in west threaten to leave Gaddafi bottled up in Tripoli

REBEL advances in Libya's Western Mountains have opened the possibility of an encirclement of the capital, Tripoli, cutting off Colonel Muammar al-Gaddafi from the bulk of his forces elsewhere in the country.

Such an encirclement would leave Gaddafi isolated in his capital, with the bulk of his forces cut off from supplies in positions stretching hundreds of miles to the east.

With Col Gaddafi bottled up, he would lose access to oil supplies and the rebel Transitional National Council would be free to take control of the rest of the country. Making all this possible has been a dramatic success in fierce fighting that has raged along the Jabal Nafusah, or Western Mountains, a rocky spine that juts out from the Tunisian border. The rebels claim they now control this spine and the towns that line it, advancing as far as Qalaa and Yefren (see map, right), and cutting off a series of strategic highways which had been open for his forces.

Meanwhile to the east, rebel successes in Misrata have transformed the city from one under siege to a springboard for attacks deep into Col Gaddafi's territory. With new weapons and fierce Nato bombing raids, the rebels have pushed their city perimeter 25 miles south of the city. Front-line commanders have told The Scotsman that they could capture the southern city of Bani Walid, cutting one of only two remaining strategic highways open to Col Gaddafi and opening up the chance of a link-up with the Western Mountain forces.

What makes this an enticing strategy for Nato is that it solves the problem of how to break a stalemate that is grinding into its third month.

For a while the rebels have now secured the besieged city of Misrata, 100 miles east of Tripoli, the rest of the coastal highway and its hinterland stretching nearly 400 miles to the rebel capital, Benghazi, is in loyalist hands.

At the weekend Apache, Gazelle and Tiger helicopters began the task of clearing this stretch, with raids against tanks and checkpoints at Brega.

But if the units here can be isolated from their supply bases, they might "die on the vine", leaving Nato to supervise what would be, essentially, a mopping-up operation.

What remains of Col Gaddafi's elite units are deployed on the highway between Misrata and Tripoli near the town of Zlitan.

A drive south, rather than west, from Misrata, would force him to divide this force. If his armour was forced to break into the open to counter a rebel thrust south, it would be prey to the fixed-wing strike aircraft and attack helicopters.

A bigger question is whether the rebels could sustain such a drive. Units such as the Black Brigade are mobile in the sense that they have fleets of pick-up trucks, painted black to distinguish them from Gaddafi's vehicles.But they lack tanks and artillery, and would find problems in overcoming even a lightly defended town without Nato air support.

The problem, in other words, would become political. Nato is already skirting the very edge of its mandate in offering combat air support to rebel units, because those units do not qualify as the civilians it was ordered by a UN resolution to protect.

To be fully effective, forward air controllers would need to be deployed with the advancing rebel units, putting further strain on a resolution that prohibits the use of foreign ground troops.

But that may be a price Nato is willing to pay to get itself out of a war it did not choose, has to fight with one arm behind its back, and which has put great strain on its cohesion.